Conventionally known photographic techniques involving use of a silver salt for image formation include a silver salt diffusion transfer reversal process.
According to the silver salt diffusion transfer reversal process, the photographic film unit generally comprises a negative sheet that is a light-sensitive material consisting mainly of silver halides and a positive sheet. After the negative sheet is exposed to light, the exposed emulsion surface is brought into contact with a positive sheet, and the combined unit is immersed in a developing solution and squeezed by rollers to press excessive solution out. After allowing the unit to stand for a certain period of time, silver halides of the negative sheet in the sufficiently exposed areas are developed by the action of the developing solution, while silver halides in the unexposed areas are solubilized by the action of a silver halide solvent and are thereby diffused and transferred into the positive sheet, in which the solubilized silver halides are reduced to silver to form a positive image.
The above-described technique gives rise to problems ascribed to the use of a developing solution containing an alkali, such as contamination of hands or clothes and corrosion of devices. In addition, this method requires skill in handling or maintenance of chemicals for the developing solution.
In order to overcome these disadvantages, various systems for easily forming positive images only by heat have been proposed.
It is considerably difficult to solubilize silver halides only by heating in the presence of a silver halide solvent. Therefore, solubilization of silver halides was attempted by using silver halides containing at least 80 mol % of silver chloride, as disclosed, e.g., in Japanese Patent Publication No. 18667/66. However, emulsions comprising silver chloride for the most part are generally low in sensitivity and are not sufficiently applicable to a practical use.